The overall goals of this proposal are to study the mechanisms of hearing and sound communication in a simply organized vertebrate, the oyster toadfish (Opsanus tau). The toadfish is a vocal species whose acoustic vocalization behavior, vestibular organs, and brainstem nuclei are well studied. In fishes, sound is transduced by hair cells of one or more otolith organs that are adapted as acoustic receivers operating in a frequency range up to several kHz. The otolith organs of fishes have been important preparations for the study of hair cell synaptic physiology, efferent mechanisms, hair cell biophysics, and neurophysiology-behavior relations in hearing. However, wide gaps in our understanding of hearing and sound communication mediated by otolith organs has been due to the difficulties in specifying the adequate stimulus (pressure, displacement, velocity or acceleration), and the identify of the endorgans normally responsive to these quantities. New methods for controlling these variables now make it possible to advance our knowledge about the mechanisms of the ears, hearing, and sound communication in these simply organized vertebrates. Sounds, including sinusoids and model toadfish vocalizations will be synthesized in the laboratory using a 3-dimensional motion system and a sound pressure source. Recording from primary afferents of the utricle, lagena, and saccule will characterize their spatial directionality, acceleration and pressure sensitivity, and tuning. Comparisons with the vocalization sounds toadfish normally experience in the natural environment will permit an identification of the adequate stimulus in the field, and the role of each endorgan in hearing. Intracellular recording of primary afferents from the otolith organs will physiologically characterize their response to communication sounds, and injections of biocytin or other tracers molecules will allow visualization of their peripheral terminations and central projections. New knowledge will be obtained on the morphological substrates that distinguish acoustic and vestibular functions peripherally and centrally, and the projection and convergence from the various endorgans of auditory signals of known biological significance in the brainstem will be described and analyzed. Studies of hearing in animals are not only necessary for the development of specific model systems for normal and pathological hearing functions in humans, but are also important for providing a general biological and evolutionary context within which new data from any species, including humans, may be better interpreted and applied. This research will help determine the distinctions and commonalities between vestibular and auditory functions and structures so that we may better understand how vestibular organs may function in hearing.